Compressor anti-surge control for a gas turbine engine



March 12, 1963 H. J. WOOD 3,

COMPRESSOR ANTI-SURGE CONTROL FOR A GAS TURBINE ENGINE Filed Feb. 5,1959 5 Sheets-Sheet 1 TO OTHER AIR RS Fl 61 INVENTOR. 26 HOMER J.WOOD

ATTORNEYS March 12, 1963 H. J. WOOD 3,080,712

COMPRESSOR ANTI-SURGE CONTROL FOR A GAS TURBINE ENGINE Filed Feb. 5,1959 5 Sheets-Sheet 2 FiG.3.

Consmni Speed Lines INVENTOR. w J'' Q I43 HOMER J. wooo CompressorAirflow BY MQAQJ;

ATTORNEYS March 12, 1963 H. J. WOOD 3,080,712

COMPRESSOR ANTI-SURGE CONTROL FOR A GAS TURBINE ENGINE Q Filed Feb. 5,1959 5 Sheets-Sheet 3 FIGS.

Constant Speed Lines Sens or Pa rumeter a/ t Compressor Pressure RatioFIG.6.

INVENTOR. HOMER J.WOOD

ATTOR NEYS March 12, 1963 H. J. WOOD 3,080,712

COMPRESSOR ANTI-SURGE CONTROL FOR A GAS TURBINE ENGINE Filed Feb. 5,1959 5 Sheets-Sheet 4 INVENTOR.

HOMER J. W0 OD BY %A 1 J51?! 1 ATTORNEY COMPRESSOR ANTI-SURGE CONTROLFOR A GAS TURBINE ENGINE 5 Sheets-Sheet 5 Filed Feb. 5, 1959 FIG.9.

I1 I w l KJI a INVENTOR.

HOMER J.WOOD

ATTORNEYS Patented Mar. 12, 1963 3,080,712 COMPRESSUR ANTI-SURGE CONTROLFGR A GAS TURBINE ENGENE Homer J. Wood, Sherman Oaks, Califi, assignorto Continental Aviation and Engineering Corporation, Detroit, Mich, acorporation of Virginia Filed Feb. 5, 1959, Ser. No. 791,447 18 Claims.(Cl. 641-3919) This invention relates to control systems for gas turbineengines, and more particularly to a pneumatic surge control sensorsystem operable to prevent compressor surge by discharging controlledamounts of compressor airflow.

Compressor surge in gas turbine engines results in violent pulsations ofgas flow and pressure. These upset the operating cycle to the extentthat gross deterioration of engine performance immediately occurs, andexcessive gas temperatures are likely to result. In addition, thepressure variations are so violent that physical damage to the enginemay result. Although it is possible to build engines which can withstandsurge conditions, or which develop only mild pressure pulsations,undesirable increases in weight and cost are involved. It is a furthercharacteristic of many gas turbines that the most efficient operatingpoint occurs quite close to the compressor surge range, and to avoidsurging difiiculties by moving the operating point results inundesirable deterioration of performance.

For gas turbine engines used primarily for compressing air for use byother equipment, certain special problems are involved in addition tothose encountered in other gas turbines with respect to compressor surgedifliculties. In particular, such engines must face very abrupt shutoilof the compressed air demand flow. This results in the very rapidestablishment of a condition tending to produce surge. This condition isoften called shock surge, in recognition of the shock efie'ct of asudden shut-oil of air delivery.

Heretofore, little has been accomplished in finding an efiective, yetinexpensive and simplified method of avoiding compressor surgeconditions.

An object of the present invention is to improve performance of gasturbine engines by preventing compressor operation from reaching surgeconditions.

Another object of the invention is to control gas turbine engineoperation by providing a system for controlling compressor pressureratios and airflow.

A further object of the invention is to prevent compressor surge in agas turbine engine by providing a new pressure sensitive control systemoperable to discharge controlled amounts of compressor airflow.

Yet another object of the invention is to simplify gas turbine enginecontrols by providing an automatically operating compressor surgeprevention system.

Still a further object of the invention is to prevent compressor surgeconditions in a gas turbine engine by providing an automatic pressuresensitive pneumatic valve system operated through a surge sensor whichis responsive to compressor operation.

Yet a further object of the invention is to increase the operatingefficiency of gas turbine compressors by providing a control systemenabling the compressor to operate at the highest effective pressureratio at all operating ranges.

Still another object of the invention is to prevent surge conditions ingas turbine engines by providing a new pressure sensitive control systemoperable to modify fuel flow to the engine.

For a more complete unedrstanding of the invention, reference may be hadto the accompanying drawings illustrating a preferred embodiment of theinvention in which like reference characters refer to like partsthroughout the several views and in which FIG. 1 is a diagrammatic viewof a preferred control system embodying the invention as applied to apreferred gas turbine engine.

FIG. 2 is a longitudinal cross-sectional view of a preferred pneumaticsensor construction as embodied in the system of FIG. 1.

FIG. 3 is a longitudinal cross-sectional view of a preferred servo valveconstruction as embodied in the system of FIG. 1.

FIG. 4 is a compressor map for purposes of explaining the surge controlconcept of the invention.

FIG. 5 is another compressor map utilizing different parameters.

FIG. 6 is a diagram illustrating the aerodynamic relationship of twoorifices in series.

FIG. 7 is a diagrammatic cross-section of a portion of a chamber in thesensor of FIG. 2.

FIG. 8 is a diagrammatic view of another preferred modification of theinvention using the sensor of FIG. 2

for controlling engine fuel flow.

FIG. 9 is a cross-sectional view of a preferred fuel control valve asused in the modification of FIG. 8.

In the embodiment of the invention represented in FIG. 1 a preferred gasturbine engine 10 is illustrated as comprising a housing 11 having anair inlet 12, a combustion chamber or combustor 13, an exhaust nozzle14, and a compressor air chamber 15, the housing 11 supporting a turbine16 and a compressor 17 operable to supply compressed air to the chamber15, the compressed air being used in the turbine and also stored forselective delivery through a discharge pipe 18 having a butterfly valve18a for delivery to any compressed air consumer. It will be noted thatthe compressor is operable to deliver more air than is consumed by theturbine 16. Connected with the discharge pipe 18 is a bypass pipe 19having a discharge stack 20 for exhausting excess compressed air ascontrolled by a servo valve 21, shown in more detail in FIG. 3 as willbe described later.

The servo valve is generally controlled by a surge control sensor 22illustrated more fully in FIG. 2, as will also be described later.Suitable pressure probes and communicating pressure conduits are alsoprovided.

In order to fully comprehend the function and operation of the presentsystem, an understanding of the known theoretical operatingcharacteristics of an air compressor, such as is utilized in a gasturbine engine, is first necessary. FIG. 4 illustrates, generally, acompressor map on which operation of any compressor may be diagrammed interms of corrected compressor airflow and compressor pressure ratio,where:

P /P is the pressure ratio across the compressor; that is, compressordischarge pressure (P per compressor inlet total pressure (P and W,,/0/6 is corrected compressor airflow, where W,, is uncorrectedcompressor airflow, 0 is ambient temperature correction to NACA std.,and 5 is barometric correction to NACA std.

Note-In gas turbine engines not in motion, as where used solely tocompress air for use by other equipment, compressor inlet total pressurewould be equivalent to atmospheric pressure.

With compressor operation described in terms of corrected parameters,the compressor map of FIG. 4 then fixes compressor performance for allinlet conditions and enables one to predict compressor performance forvarying inlet conditions.

Compressor surge is a phenomenon associated with the operation of allcompressors. For example, assuming NACA standard ambient temperature andbarometric conditions, a compressor operating at a given speed,regardless of its power source, to deliver a certain airflow W at somegiven pressure ratio P /P (point 1 on the map), and a large air valvedownstream of the compressor, a partial closing of the air valve whilemaintaining compressor speed will reduce compressor airflow W and theoperating point of the compressor will shift to point (2) on the map.Upon continued closing of the air valve and consequent reduction ofairflow, the compressor will reach a state of unstable operation,commonly called compressor surge, and on the compressor map willsubstantially occur when the operating point reaches a line such as thesolid line of FIG. 4. To the left of this line is the unstable operatingline, and to the right will be seen typical lines of ditferent constantspeeds of the compressor.

With the compressor as part of the compressed air producing gas turbineengine 10 shown in FIG. 1, the combustor 13 and turbine 16 will act onthe compressor 17 in a similar manner asthe theoretical valve mentionedabove; for example, at a given speed, held by any means such as agovernor (not shown), an increase in fuel flow to the combustor 13 willraise the temperature of the air (and consequently its volume) andthereby act to decrease compressor air flow, moving the operating pointon the compressor map of FIG. 4 toward the surge line. Opening of thebutterfly valve 18a would increase compressor air flow, moving theoperating point on the compressor map away from the surge line. Closingof the butterfly valve 18a would again decrease compressor air flow,again moving the operating point on the compressor map toward the surgeline.

It will be apparent, then, that a control system operable to bleed offthe proper controlled amounts of air will function to maintaincompressor operation somewhere to the right of the surge line, andpreferably no closer to the surge line than a predetermined value, suchas is illustrated by a dash line in FIG. 4. In other words, the controlsystem will function in such a manner that any tendency of thecompressor to operate at, Surge will immediately be offset by increasingcompressor airflow so that the operating point of the compressor willremain to the right of the surge line of FIG. 4.

Thus it is seen that corrected compressor airflow is the criticaldetermining factor with which the control system is concerned. Inseeking to develop a suitable control system, it was found thatcorrected compressor airflow could be sensed by measuring static andtotal pressure at the inlet to the compressor.

Mathematically,

where P zCompressor inlet total pressure P =Compressor inlet staticpressure W =Compressor airflow =Ambient temperature correction to NACAstd. 6=Barometric correction to NACA std.

In other words, for any given value of corrected compressor airflowthere exists One and only one value of the compressor inlet ratio P -P tThus it is possible to construct a compressor map using P -P Pt This mapis illustrated in FIG. 5, and illustrates the concept that it will bepossible by making a mechanical com parison of compressor pressures, toestablish an operating or modulation line (dotted line in FIG. 5), sincein place of for any operation of the compressor.

Referring to FIGS. 1, 2 and 3, the preferred control system comprisesthe sensor 22 and the servo 21, a compressor inlet total pressure (Psensing conduit 25, a compressor inlet static pressure (P sensingconduit 26, and a compressor discharge pressure (P sensing conduit 27,the signal from the sensor 22 to the servo 21 being transmitted througha signal pressure (S) conduit 28.

The sensor 22 preferably comprises a multiple part housing 29 having asensor chamber 30 divided by a compressor inlet pressure sensingdiaphragm 31 of area A a second sensor chamber 32 divided by acompressor pressure ratio sensing diaphragm 33 of area A and a controlchamber 34. The diaphragms 31 and 33 are connected to a control pin 35operable to control the opening of a port 36, connecting the controlchamber 34 with the signal pressure (S) conduit 28, in proportion to themagnitude of pressure area differential existing across the stackeddiaphragms 31 and 33. The pin 35 is preferably biased toward the closedposition by a relatively weak spring 40 hearing between a plate 41,adjustable by any means such as an adjusting screw 42, and a plate 43connected to the control pin 35. For purposes of the followingexplanatiointhe small force of the spring 40 will not be included.

One side of the chamber 30 is openly connectedthrough a port 44 to abranch 25a of the conduit 25, and the other side of the chamber 30 isopenly connected through a port 45 with the conduit 26, suchthat thediaphragm 31 will be responsive to the diiferential between compressorinlet total pressure P, and static pressure P to a degree dependent onthe diaphragm area A One side of the chamber 32 is openly connectedthrough a port 46 with another branch 25b of the conduit 25. The otherside of the chamber 32 is subject to an intermediate pressure Pdeveloped between series-connected orifices 47 (openly connected withthe conduit 27) and 48 (openly connected with a third branch 250 of theconduit 25).

The area a of the orifice 48 is preferably adjustable by means of aneedle valve 49, while the area a of the orifice 47 is preferably fixed.Thus the diaphragm 33 will be responsive to the differential betweencompressor inlet total pressure P, and the intermediate pressure P to adegree dependent on the diaphragm area A It can be shown that the sensoroperation is in accord with the conditions required by the compressormap FIG. 5; that is, if properly constructed, the sensor will axiallyactuate the pin to open and close the port 36 in correspondence to theparameter aphragms 31 and 33, the force balance equation (P,P )A (P P )Adescribes the function of the sensor 22. Dividing by P, and A t t A2Since any constant may be selected for the ratio it remains only to showthat e) Pt 1 P,

t on, 5 P2 2 P,

age e a P. P, 2 Pt 3 P,

By choosing the proper values of a and a ea Pt P.

In constructing the sensor 22,

can be made to equal so that 1 and are selected so that, for values ofit is possible to determine the value of that the sensor will regulateto, which will be the selected modulation line in FIG. 5.

In accomplishing this function, the servo Valve 21, shown in FIG. 3,operates to bleed excess compressor air from the by-pass pipe 19 throughthe stack 20 in response to the signal from the sensor 22.

The servo valve preferably comprises a housing 55 having an inletchamber 56 openly connected with the bypass pipe 19 and terminating inan annular valve seat 57. A poppet valve 58 having an annular closurememher 57 adapted to seat on the valve seat 57 is positioned in acontrol chamber 60. A diaphragm 61 is secured to the poppet valve 53 andthe housing 55 so that the valve 58 will operate axially as determinedby a pressure differential existing between the chambers 56 and 60 toopen and close the valve. A tube element 62 is carried by the poppetvalve 58 and slides axially within a sleeve 63 supported by the housing55. The sleeve 63 carries a needle valve element 64 which extends intothe end of the tube element 62, acting to open and close an orifice 65as determined by the position of the poppet valve 58. The tube 62 thusvariably communicates the inner end of the chamber 60 with the dischargepressure P of the compressor. The chamber 60 communicates at all timeswith the conduit 28 which is connected with the sensor 22 and transmitsthe sensor signal S.

In substance, when the sensor valve port 36 is open, pressure in thechamber 60 is bled out through the conduit 28 relativeto the degree towhich the sensor signal port 36 is opened by the pin 35 (see FIG. 2),permitting the poppet valve 58 to open and bleed compressor dischargeair out the stack 20. As the poppet valve 58 opens, however, the area ofthe orifice 65 will increase to bleed compressor discharge pressure Pthrough the tube 62 into the chamber 6t) tending to close the poppetvalve 58, the eifective area on the chamber 60 side of the diaphragm 61being greater than the effective area on the chamber 56 side as shown.

In efiect, the pressure existing in the chamber 66 will take anintermediate pressure valve between S and P which is a function of thedegree to which the sensor port 36 is opened. Consequently, the positionof the poppet valve 58 in the servo is a function of the bleed area ofthe port 36 in the sensor, Which in turn, due to the pressuredifferential existing across the stack diap-hragms of the sensor 22,will take on values such as to maintain in accordance with thediscussion given previously, tending to maintain operation of thecompressor on the control line shown in the compressor map of FIG. 4.

It will be apparent that when the butterfly valve 1? (FIG. 1) is openedfor delivery of compressed air to the user, compressor airflow willincrease, the surge sensor 22 will operate to cut-off bleed through theport 36, the poppet valve 58 of the servo valve 21 will close fully, andno air will bleed through the by-pass pipe 19. Thus the compressor willbe able to operate to the right of the control line of FIG. 4 to producethe required airflow. However, as the butterfly valve 18a is closed,compressor airflow decreases, and when an imbalance is sensed by thesensor 22, indicating an approach toward surge conditions, the sensorport 36 will open to bleed the servo chamber 69 and open the poppetvalve 58 to the necessary degreeas described above.

The servo valve 21 also preferably performs the additional function ofpreventing shock surge due to rapid closing of the buterfiy valve 18a.For most purposes, a compressor delivery valve such as that illustratedshould be operable to move from full open to full closed in about 0.2second. A second control chamber 70 is provided in the servo valvehousing 55, and is divided by a diaphragm 71 which carries a needlevalve element 72 extending into a port 73 which communicates the chamber60 with atmosphere via a passage 74. A conduit 75 connects with apassage 76 in the housing 55, the passage 76 opening .to the inner endof the chamber 70.

' The conduit 75 is connected, as shown in FIG. 1, with the pipe 18upstream of the butterfly valve 18a. Upon suddent closing of thebutterfly valve, an immediate presvalve 58 as previously described,providing for an immediate increase in compressor airflow to preventsudden operation toward the surge line of the compressor map FIG. 4,which the sensor 22 would not have time to counteract. A passage 77connects the passage 76 with the outer end of the chamber 70, thepassage 77 being provided with an adjustable orifice 78 so that theincreased pressure P will be subsequently transmitted to the outersideof the diaphragm 71 to shortly equalize pressures and permit thediaphragm 71 to be urged inward by a spring 79 or other means to closethe port 73 as the sensor 22 takes over control of the poppet valve 58.

Although it is practical to use a control system of the general ideaillustrated in FIG. 1 for shaft power or jet engines, a more likelymodification utilizes the basic surge sensor 22 to modulate a fuel servovalve rather than an air valve. In FIG. 8 a preferred gas turbine jetengine 85 illustrating such a modification comprises a housing 86 havingan air inlet 87, a combustion chamber or combustor 88, a jet exhaustnozzle 89, and a compressed air chamber 90, the housing 36 supporting aturbine 91 and a compressor 92 operable to supply compressed air to thechamber 90 for use in the combustor 88 only.

The sensor 22 is the same as that previously described, as well as thecompressor inlet total pressure ('P sensing conduit 25 with its branches25a, 25b and 25c, the compressor inlet static pressure (P sensingconduit 26, the compressor discharge pressure (P sensing conduit 27, andthe sensor signal pressure conduit 28. However, the conduit 28 issuitably connected to and transmits the sensor signal to a fuel servovalve 93 which is preferably constructed on the lines suggested in FIG.9, having a fuel inlet 94 suitably connected with the supply of a fuelpump (not shown) and a fuel outlet 95 delivering metered fuel to theengine, the fuel flowing through passages 96 and 97 in the valve 93 andmodulated by means of a poppet valve 98.

The valve 93 is provided with a pair of compartments 100 and 101respectively divided by diaphragms 102 and 103 into respective controlchambers 164, 105, 106 and 107. Chamber 104 is connected by a passage110 with the fuel passage 97, chambers 105 and 107 are connectedrespectively by passages 1 1-1 and 112 with the sensor signal pressureconduit 28, and chamber 106 is connected by a passage 113 with a branchconduit 25d sensing compressor inlet total pressure P, from the conduit25. A conduit 27a connects the passages 111 and 112 with the compressordischarge pressure (P sensing conduit 27, and includes a restrictedorifice 114.

The poppet valve 98 is operably connected by means of a pin 115 with thediaphragm 102, being urged toward the closed position by a relativelyweak spring 116. The two diaphragms 102 and 103 are operably connectedin series by means of a pin 117.

It will be apparent that the diaphragm 102 balances fuel pressureagainst a pressure intermediate the surge sensor signal pressure (S) andthe compressor discharge pressure (P The diaphragm 103 balancescompressor inlet total pressure against the same intermediate pressure.In essence, the position of the poppet valve will be determined by thesurge sensor signal pressure which varies relative to the opening of thesensor valve port 36.

Thus, when the compressor approaches surge conditions, the sensor 22operates as previously explained to open the valve port 36. Thisdecreases the pressure in the fuel valve chambers 105 and 107 and thepoppet valve 98 moves toward a closed position to reduce fuel flow.Thermodynamically, reduced fuel flow to a turbine engine operates toreduce compressor discharge pressure and hence increases compressorainflow, and the operating point on the compressor map of FIG. 4 willmove away from the surge line. Thus the result of reducing fuel flow isin substance, as far as its effect on the engine is 8 concerned, thesame as bleeding off excess compressed air as is done in the firstmodification described.

As the fuel poppet valve 98 closes, the fuel pressure in the chamber 104will decrease correspondingly, so that, since the diaphragm 102 balancesfuel pressure with a pressure depending on surge signal pressure (S),the poppet valve 98 will seek a stable position to provide that fuelflow which will not cause surge conditions.

It is apparent also that the present system will prevent excess fuelflow during acceleration. Since increased fuel flow on accelerationcauses a rise in the temperature of the combustion mixture, its volumealso increases, resulting in a decrease of compressor airflow. The surgesensor, as previously explained, operates to open the valve port 36 onsuch a decrease as the operating point moves toward surge conditions,and this again will cause the fuel poppet valve 93 to move toward aclosed position to decrease fuel flow.

Although I have described only two preferred embodiments of myinvention, it will be apparent to one skilled in the art to which theinvention pertains that various changes and modifications may be madetherein without departing from the spirit of the invention or the scopeof the appended claims.

I claim:

1. A control system for a gas turbine engine having a fuel system and anair compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, an air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operating of said servo means in response tovarying compressor airflow, said sensing means comprising an actuatingmeans operably connected with said servo means and a pressure responsivemeans adjusting said actuating means in response to changes in thebalance of two pressure differentials, one of said pressuredifferentials being the diflerential between compressor inlet totalpressure and compressor inlet static pressure, the other of saidpressure differentials being the differential between compressor inlettotal pressure and a pressure intermediate compressor discharge pressureand compressor inlet total pressure.

2. A control system for a gas turbine engine having a fuel system and anair compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, an air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operation of said servo means in response tovarying compressor airflow, said sensing means comprising a housinghaving two pressure sensitive elements, a movable actuator operablyconnecting said pressure sensitive elements with said ser-vo means, oneof said pressure sensitive elements moving said actuator in response tochanges in pressure differential between a compressor inlet totalpressure and compressor inlet static pressure, and the other of saidpressure sensitive elements moving said actuator in response to changesin pressure differential between compressor inlet total pressure and apressure intermediate compressor total pressure and compressor dischargepressure.

3. The control system as defined in claim 2 and in which said pressuresensitive elements are balanced in accordance with the force-balanceequation t 0 2 equals b"" t) 1 in which P represents compressor inlettotal pressure, P, represents pressure-inlet static pressure, Prepresents the pressure intermediate compressor total pressure andcompressor discharge pressure, A represents the total effective area ofthe first mentioned pressure sensitive element, and A represents thetotal efifective area of the second mentioned pressure sensitiveelement.

4. A control system for a gas turbine engine having a fuel system and anair compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, an air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operation of said servo means in response tovarying compressor airflow, said sensing means comprising a housinghaving a first pressure chamber and a second pressure chamber, a movablepressure sensitive element disposed in each of said chambers and eachelement having a predetermined total effective area, a movable actuatoroperably connecting said pressure sensitive ele ments with said servomeans, a compressor inlet total pressure sensing means, a compressorinlet static pressure sensing means, and a compressor discharge pressuresens ing means, means connecting said first pressure chamber on one sideof the pressure sensitive element with said total pressure sensing meansand means connecting said first pressure chamber on the other side ofthe pressure sensitive element with said static pressure sensing means,means connecting said second pressure chamber on one side of thepressure sensitive element with said total pressure sensing means, meansconnecting said second pressure chamber on the other side of thepressure sensitive ele ment with said total pressure sensing means andincluding a restricted orifice having a predetermined area, and meansconnecting said second pressure chamber on said other side of thepressure sensitive element with said discharge pressure sensing meansand including a second restricted orifice, the pressure in said otherside of the last mentioned pressure sensitive element being theintermediate pressure measured between said orifices and dependent onthe ratio of the areas of said orifices.

5. The control system as defined in claim 4 in which said pressuresensitive elements comprise axially movable diaphragm members, and saidactuator axially connecting said diaphragm members in series.

6. The control system as defined in claim 4 and having means variablyadjusting the area of one of said orifices to vary said intermediatepressure and thereby adjust the operating pressure differential actingon the last mentioned pressure sensitive element.

7. A control system for a gas turbine engine haViIlg a fuel and an aircompressor characterized by a predictable tendency to develop compressorsurge conditions at operating ranges producing a predetermined reductionof compressor airflow, an air inlet and an air discharge for saidcompressor, said control system comprising first means variablyincreasing compressor airflow to avoid said surge conditions a servomeans operating said first means and a sensing means automaticallycontrolling operation of said servo means in response to varyingcompressor airflow, said servo means comp-rising a housing having ableed air outlet, an air inlet openly con nected with said compressordischarge, a bleed valve means operable to bleed controlled amounts ofcompressed air from said air outlet and disposed intermediate saidoutlet and inlet, a pressure control chamber, and a pressure sensitiveelement operably connected with said bleed valve and disposedintermediate said inlet and said control chamber, said pressuresensitive element actuating said bleed valve in response to changes inpressure differential between compressor discharge pressure and thepressure in said control chamber, and said sensing means comprisingmeans transmitting a variable pressure to said control chamber relativeto changes in compressor airflow.

8. The control system as defined in claim 7 and in which said servomeans includes means variably openly connecting said control chamber tocompressor discharge pressure, said last mentioned means being operablerelative to bleed valve operation to modify relative to the position ofsaid bleed valve the control chamber pressure as transmitted from saidsensing means.

9. A control system for a gas turbine engine having a fuel system and anair compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, an air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operation of said servo means in response tovarying compressor airflow, and a second sensing means automaticallyoperating said servo means in response only to a sudden decrease incompressor airflow.

10. The control system as defined in claim 9 and in which said servomeans com-prises an air bleed valve means operable to bleed controlledamounts of compres-sed air from said air outlet and pressure responsivemeans operably connected with said air bleed valve means, said secondsensing means comprising a variable pressure transmitting meansactuating said pressure responsive means only upon a sudden decrease incompressor airflow.

11. The control system as defined in claim 10 and in which said pressureresponsive means comprises a pressure sensitive element actuating saidair bleed valve in response to changes in a pressure diiferentialbetween compressor discharge pressure and a pressure intermediatecompressor discharge pressure and the pressure of said pressuretransmitting means.

12. The control system as defined in claim 9 and in which said servomeans comprises a housing having a bleed air outlet, an air inlet openlyconnected with said compressor discharge, a bleed valve means operableto bleed controlled amounts of compressed air from said air outlet anddisposed intermediate said outlet and inlet, a pressure control chamber,and a pressure sensitive element operably connected with said bleedvalve and dis posed intermediate said inlet and said control chamber,said pressure sensitive element actuating said bleed valve in responseto changes in pressure diflerent-ial between compressor dischargepressure and the pressure in said control chamber, and said secondsensing means comprising means transmitting a variable pressure to saidcontrol chamber relative only to a sudden decrease in compressorairflow.

13. The control system as defined in claim 12 and in which said secondsensing means comprises a housing having a pressure chamber, a valvemeans variably connecting said servo means pressure control chamber withatmosphere, a pressure sensitive element operably connected with saidvalve means and disposed in said pressure chamber, means openlyconnecting said pressure chamber on one side of said pressure sensitiveelement with compressor discharge pressure such as will open said valvemeans upon increase of compressor discharge pressure, and a second meansopenly connecting said pressure chamber on the other side of saidpressure sensitive element with compressor discharge pressure such aswill equalize pressure on said pressure sensitive element, said secondmeans including a restricted orifice operable to delay the aforesaidpressure equalization only after a sudden change of compressor dischargepressure and to permit the aforesaid pressure equalization upon othermore gradual changes of compressor discharge pressure.

14. A control system for a gas turbine engine having a fuel system andan air compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, and air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operation of said servo means in response tovarying compressor airflow said sensing means comprising an actuatingmeans operably connected with said servo means and a pressure responsivemeans adjusting said actuating means in response to changes in thebalance of two pressure differentials one of said pressure differentialsbeing the differential between compressor inlet total pressure andcompressor inlet static pressure, the other of said pressuredifferentials being the differential between compressor inlet totalpressure and a pressure intermediate compressor discharge pressure andcompressor inlet total pressure, said first means comprising a fuelcontrol valve means operable to vary fuel flow to said engine andpressure responsive means actuated by said servo means and operablyconnected with said fuel control valve means whereby compressor airflowwill be varied by varying fuel flow to said engine.

15. The control system as defined in claim 14 and in which said lastmentioned pressure responsive means comprises a pressure sensitiveelement actuating said fuel control valve means in response to changesin a pressure differential between fuel pressure and a pressureintermediate compressor discharge pressure and the pressure of saidfirst mentioned pressure responsive means.

16. The control system as defined in claim 15 and in which the aforesaidfuel pressure is taken downstream of said fuel control valve means.

17. The control system as defined in claim 16 and in which said secondmentioned pressure responsive means includes a second pressure sensitiveelement modifying actuation of said fuel control valve means in responseto changes in pressure differential between compressor inlet totalpressure and the aforesaid intermediate pressure.

18. A control system for a gas turbine engine having a fuel system andan air compressor characterized by a predictable tendency to developcompressor surge conditions at operating ranges producing apredetermined reduction of compressor airflow, and air inlet and an airdischarge for said compressor, said control system comprising firstmeans variably increasing compressor airflow to avoid said surgeconditions a servo means operating said first means and a sensing meansautomatically controlling operation of said servo means in response tovarying compressor airflow said sensing means comprising an ac tuatingmeans operably connected with said servo means and a pressure responsivemeans adjusting said actuating means in response to changes in thebalance of two pressure differentials one of said pressure differentialsbeing the differential between compressor inlet total pressure andcompressor inlet static pressure, the other of said pressuredifferentials being the differential between compres-sor inlet totalpressure and a pressure intermediate compressor discharge pressure andcompressor inlet total pressure, said first means comprising a housinghaving a fuel inlet and a fuel outlet, a fuel control valve operable tovary fuel flow to said engine and disposed intermediate said inlet andoutlet, a pressure control chamber, a pressure sensitive elementoperably connected with said fuel control valve and disposed in saidpressure control chamber, and means openly connecting said fuel outletwith said control chamber on one side of said pressure sensitiveelement, said servo means being operable to transmit a variable pressureto said control chamber on the other side of said pressure sensitiveelement relative to changes in compressor airflow.

References Cited in the file of this patent UNITED STATES PATENTS2,463,865 Gilfillan Mar. 8, 1949 2,618,431 Walker Nov. 18, 19522,645,240 Drake July 14, 1953 2,767,725 Long Oct. 23, 1956 2,813,672Long et a1. Nov. 19, 1957 2,846,846 Mock Aug. 12, 1958 2,851,855 GambleSept. 16, 1958 2,858,700 Rose Nov. 4, 1958 2,863,601 Torell Dec. 9, 19582,886,968 Johnson May 19, 1959 em mam

14. A CONTROL SYSTEM FOR A GAS TURBINE ENGINE HAVING A FUEL SYSTEM ANDAN AIR COMPRESSOR CHARACTERIZED BY A PREDICTABLE TENDENCY TO DEVELOPCOMPRESSOR SURGE CONDITIONS AT OPERATING RANGES PRODUCING APREDETERMINED REDUCTION OF COMPRESSOR AIRFLOW, AND AIR INLET AND AN AIRDISCHARGE FOR SAID COMPRESSOR, SAID CONTROL SYSTEM COMPRISING FIRSTMEANS VARIABLY INCREASING COMPRESSOR AIRFLOW TO AVOID SAID SURGECONDITIONS A SERVO MEANS OPERATING SAID FIRST MEANS AND A SENSING MEANSAUTOMATICALLY CONTROLLING OPERATION OF SAID SERVO MEANS IN RESPONSE TOVARYING COMPRESSOR AIRFLOW SAID SENSING MEANS COMPRISING AN ACTUATINGMEANS OPERABLY CONNECTED WITH SAID SERVO MEANS AND A PRESSURE RESPONSIVEMEANS ADJUSTING SAID ACTUATING MEANS IN RESPONSE TO CHANGES IN THEBALANCE OF TWO PRESSURE DIFFERENTIALS ONE OF SAID PRESSURE DIFFERENTIALBEING